In recent years, organic light-emitting diodes have been attracting attention as a technology for next-generation displays and solid state lighting. In the early study of organic light-emitting diodes, fluorescence has been mainly used. However, in recent years, an organic light-emitting diode utilizing phosphorescence which exhibits higher internal quantum efficiency has been attracting attention.
Mainstream of emissive layers utilizing phosphorescence in recent years are those in which a host material containing an organic material is doped with an emissive metal complex including iridium or platinum as a central metal.
However, an iridium and platinum are rare metals and are therefore expensive, giving rise to the problem that organic light-emitting diodes using these rare metals are increased in cost. Copper complexes, on the other hand, likewise emit phosphorescent light and are inexpensive, so that they are expected to reduce the production cost.
An organic light-emitting diode using a copper complex as a light-emitting material has been disclosed, but the organic light-emitting diode has a problem of a high turn-on voltage. In order to attain energy saving and long lifetime of the organic light-emitting diode, it is preferable to use an emitting dopant which starts emission with a lower applied voltage.